Discharge lamp lighting device

ABSTRACT

A discharge lamp driving device capable of detecting a lamp life end reliably in a high or low temperature environment for circuit protection, yet preventing the occurrence of the cataphoresis phenomenon. Impedance elements Z 1  and Z 1  are inserted respectively between one filament ends of individual discharge lamps La 1  and La 2  and a node (the ground) having no high frequency amplitude in order to detect a difference between AC components of individual lamp voltages VLa 1  and VLa 2  in closed loops of the discharge lamps La 1  and La 2  and the impedance elements Z 1  and Z 1  in order to judge whether or not the depletion of the emitter occurs. Thus, it is possible to reliably judge the presence of abnormality even when the amplitudes of the lamp voltages VLa 1  and VLa 2  varies in a range of low to high temperature. Also, since there is no need to provide a DC blocking capacitor to a secondary winding N 2  of a leakage transformer LT 1,  the discharge lamps La 1  and La 2  can be free from the DC component so as to be prevented from causing the cataphoresis phenomenon.

TECHNICAL FIELD

[0001] The present invention is directed to a discharge lamp drivingdevice with an abnormality detection and protection function ofdetecting a lamp life end for circuit protection.

BACKGROUND ART

[0002] (First Prior Art)

[0003]FIG. 1 s a circuit diagram showing one example of a priordischarge lamp driving device which is identical in circuitconfiguration to that shown in FIG. 36 of Japanese Patent PublicationNo. 8-251942. A rectifier DB composed of a diode bridge is connected toan AC power source AC through a surge absorption element ZNR and afilter circuit F. Connected across a pulsating output terminals are ahigh frequency bypassing capacitor C2, a series combination of switchingelements Q1 and Q2 in the form of field effect transistors through aseries circuit of diodes D5 and D6, a series combination of a smoothingcapacitor C10 and a diode 13, and a high frequency bypassing capacitorC11. A series circuit of an inductor L2 and a diode D12 is connectedbetween a connection point of switching elements Q1 and Q2 and aconnection point of smoothing capacitor C10 and diode D13. A leakagetransformer LT1 has a primary winding N1 that is connected in serieswith a DC blocking capacitor C3 between the cathode of diode D5 and theconnection point of switching elements Q1 and Q2. A secondary winding N2of the leakage transformer LT1 has its one end connected through a DCblocking capacitor C9 to one of filaments of one discharge lamp La1, andhas its other end connected to one of filament of the other dischargelamp La2. The other filaments of the two discharge lamps La1 and La2 areconnected at one ends thereof to each other through an auxiliary windingN3 of the leakage transformer LT1 and a DC blocking capacitor C6. Theother ends of the filaments of the discharge lamps La1 and La2 areconnected to each other through a resonant inducing capacitor C7.Further, a harmonic distortion improving capacitor C4 is connectedacross diode D6.

[0004] The two switching elements Q1 and Q2 are driven by a controlcircuit CNT to turn on and off alternately. The leakage transformer LT1includes an auxiliary winding N4 for detection of lamp voltage of thedischarge lamps La1 and La2. The detected voltage induced at theauxiliary winding N4 is rectified by means of a diode D8 and is fed to adetection circuit 20 for detection of the lamp voltage. Based upon thusdetected lamp voltage, the control circuit CNT varies a switchingfrequency of the switching elements Q1 and Q2. In short, the source ACvoltage is rectified through rectifier DB of which pulsating output ispartially smoothed out by a valley-filling power source in the form of astep-down chopper circuit composed of switching element Q2, diode D12,inductor L2, smoothing capacitor C10 and a parasitic diode of switchingelement Q1. The partially smoothed DC output is converted into a highfrequency output by means of an inverter circuit in the form of ahalf-bridge type including the switching elements Q1 and Q2. The highfrequency output is fed through the leakage transformer LT1 to thedischarge lamps La1 and La2 as a load for driving the same. Further, inthis prior art, the harmonic distortion improving capacitor C4compensates for a voltage difference between the rectifier DB and thevalley-filling power source, while an input voltage is switched on andoff by utilization of a high frequency voltage appearing within theinverter circuit so as to draw in the input current from the rectifierDB through a resonant circuit composed of leakage transformer LT1,capacitor C3, discharge lamps La1 and La2, and capacitor C7, and throughcapacitor C4 for improving harmonic distortion of the input current. Theoperation of this prior art is known and therefore not discussed herein.

[0005] When the above prior art sees that the discharge lamps La1 or La2reaches to the lamp life end, a protective action is made as follows.That is, when the lamp reaches its lamp life end as a result of thedepletion of the negative thermion radiating material (emitter) coatedon the filaments, the lamp voltage of the discharge lamps La1 and La2increases than in a normal condition. With this result, the voltageinduced at the auxiliary winding N4 of the leakage transformer LT!increases so that the detection circuit 20 gives an abnormalitydetection signal to the control circuit CNT in response to the voltageinduced at the auxiliary winding N4 exceeds a threshold. The controlcircuit CNT responds to the abnormality detection signal for activatingthe inverter circuit to intermittently oscillate, thereby effecting aprotective action of reducing the stress on the circuit.

[0006] (Second Prior Art)

[0007]FIG. 2 shows a circuit diagram of another prior art which isidentical in configuration to the circuit disclosed in FIG. 15 of aJapanese Patent Publication 2000-100587. The second prior art differsfrom the first prior art in that the inductor L2 forming the step-downchopper circuit is omitted, that diode 12 has its anode connected to aconnection point of smoothing capacitor C10 and diode 13 and has itscathode connected to a connection point of the primary winding N1 of theleakage transformer LT1 and capacitor C3 in order to share the leakagetransformer LT1 with the step-down chopper circuit, and that an outputregulation circuit 21 is added in compensation for a largecharacteristic variation of a driving transformer T2. The outputregulation circuit 21 includes a switching element Qb realized by abipolar transistor connected across a control voltage source E through avariable resistor VR and a collector resistor Re. The switching elementQb has its base connected through a resistor Rd to a point between aresistor Rc and a capacitor Cb which are connected in series between theconnection point of the switching elements Q1, Q2 and the negative poleof the control voltage source E. Connected between the output terminalof the control circuit CNT and the negative pole of the control voltagesource E is a series combination of a diode Da, a resistor Ra, and aswitching element Qa of bipolar transistor. The switching element Qa hasits base connected through a base resistor Rb to a connection point ofcollector resistor Re and variable resistor VR. Further, a capacitor Caand a diode Db are connected in parallel across the series combinationof the switching element Qb and the collector resistor Re, while a diodeDc is connected in a base-emitter path of the switching element Qb.While the one switching element Q2 is off, capacitor Cb is chargedthrough resistor Rc so that switching element Qb is caused to turn on inresponse to the voltage increase across capacitor Cb, thereby turningoff the switching element Qa and giving no influence on the operation ofthe inverter circuit. When the switching element Q2 turns on, theswitching element Qb is turned off so that the control voltage source Eacts to charge capacitor Ca through variable resistor VR. As the voltageacross capacitor Ca increases, the switching element Qa responds to turnon, thereby causing the switching element Q2 to turn off. Accordingly,it is made possible to regulate the on-period of switching element Q2 byvarying the resistance of the variable resistor VR to thereby maintainthe output substantially at a constant level irrespective of the varyingcharacteristic of the driving transformer T2. Also this prior art hasthe same protective action as is made in the first prior art when thelamp life end is reached.

[0008] In the second prior art, however, the inclusion of the outputregulation circuit 21 brings about an asymmetry (unbalance) of theon-period of the switching elements Q1 and Q2 in the normal lampoperating condition, whereby a DC voltage will be applied to capacitorC9 connected in series with the discharge lamps La1 and La2. With thisresult, the DC voltage of the charged capacitor C9 will be superimposedupon the high frequency output of the inverter circuit in the normallamp operating, leading to a problem of causing a cataphoresisphenomenon particularly at a low temperature.

[0009] In order to solve the problem, it might be reasonable to removecapacitor C9 connected to the secondary of the leakage transformer LT1.However, this would causes another problem. That is, as the dischargelamp reaches the lamp life end, capacitor C9 accumulates an increasedvoltage so that the lamp voltage of the lamp of negative resistivityincreases to make a great difference in the lamp voltage between thenormal operating condition and the lamp life end condition. Such lampvoltage difference is utilized for detection of the lamp life end.However, in the absence of capacitor C9, the lamp voltage would makeonly a small difference between the normal operating condition and thelamp life end condition, making it difficult to detect the lamp life endparticularly at a high temperature environment.

DISCLOSURE OF THE INVENTION

[0010] The present invention has been achieved in view of the aboveproblem and has an object of providing a discharge lamp driving devicewhich is capable of detecting the lamp life end reliably at either lowor high temperature environment for circuit protection, yet preventingthe cataphoresis phenomenon.

[0011] The discharge lamp driving device in accordance with the presentinvention includes a rectifier which rectifies an AC source voltage, asmoothing capacitor which smoothes out a pulsating output of therectifier, an inverter circuit having one or more switching elements forconversion of the smoothed DC output made through the smoothingcapacitor into a high frequency output, and a load circuit including aresonance circuit and a discharge lamp and being supplied with the highfrequency output from the inverter circuit, an output transformer havinga primary connected to an output end of the inverter circuit and havinga secondary connected to one filament end of the discharge lamp, animpedance element inserted between the other filament end of thedischarge lamp and a node having no high frequency amplitude, and anabnormality detection and protection means which detects an amplitude ofthe high frequency output flowing through the discharge lamp and theimpedance element in order to make the circuit protection when thedetected amplitude exceeds a predetermined threshold.

[0012] The abnormality detection and protection means judges the lamplife end of the discharge lamp when the amplitude of the high frequencyoutput flowing through the discharge lamp and the impedance elementexceeds the threshold. Since the impedance element is inserted betweenthe other filament end of the discharge lamp and the node having no highfrequency amplitude, reliable detection of the lamp life end can be madefor the circuit protection at either low or high temperatureenvironment. Further, since there is no need to connect a capacitor onthe secondary of the output transformer, the cataphoresis phenomenon canbe prevented.

[0013] In a preferred embodiment, the impedance element is insertedbetween the other filament end of the discharge lamp and a positiveinput terminal of the inverter circuit.

[0014] The impedance element may be inserted between the other filamentend of the discharge lamp and a grounded input terminal or outputterminal of the inverter circuit.

[0015] A plurality of the discharge lamps can be connected in series onthe secondary side of the output transformer.

[0016] Each impedance element inserted between the filament of each ofthe individual discharge lamp and the node having no high frequencyamplitude is preferred to have substantially the same impedance value.

[0017] In case where the plural discharge lamps are connected in serieson the secondary side of the output transformer, the impedance elementis inserted between the other filament end of at least one dischargelamp and the positive input terminal of the inverter circuit, whileanother impedance element is inserted between the other filament end ofat least another discharge lamp and the grounded input terminal oroutput terminal of the inverter circuit.

[0018] In case where the plural discharge lamps are connected in serieson the secondary side of the output transformer, the abnormalitydetection and protection means is set to make the circuit protectiveaction when the amplitude of the high frequency output flowing throughanyone of the discharge lamps and the impedance element exceeds apredetermined threshold.

[0019] Also in case where the plural discharge lamps are connected inseries on the secondary side of the output transformer, the abnormalitydetection and protection means may be configured to detect the amplitudeof a potential at a connection point of the filaments of the pluraldischarge lamps and also detect the amplitude of the high frequencyoutput flowing through at least one discharge lamp and the impedanceelement such that it can make the circuit protective action when eitheror both of the amplitudes exceeds a predetermined threshold.

[0020] Further, in case where the plural discharge lamps are connectedin series on the secondary side of the output transformer, theabnormality detection and protection means is configured to detect theamplitude of a potential at a connection point of the filaments of theplural discharge lamps such that it makes the circuit protective actionwhen either of thus detected amplitude or the amplitude of the highfrequency output flowing through at least one of the high-voltage andlow-voltage side discharge lamps and the impedance element exceeds apredetermined threshold.

[0021] The impedance element may include a resistor, capacitor, and aseries combination of a resistor and a capacitor.

[0022] When the inverter circuit is of a self-excited type, at least aportion of a driving circuit for driving the inverter circuit can beshared with components of the abnormality detection and protectionmeans, enabling to reduce the number of the circuit components.

[0023] Still further, the impedance element can be shared with theresonance circuit included in the load circuit for reducing the numberof the circuit components.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a schematic circuit diagram showing the first prior art;

[0025]FIG. 2 is a schematic circuit diagram showing the second priorart;

[0026]FIG. 3 is a schematic circuit diagram showing a discharge lampdriving device in accordance with a first embodiment of the presentinvention;

[0027]FIG. 4 is a circuit diagram of a principal portion of the abovedevice;

[0028] FIGS. 5A-5F are waveform charts for explaining the circuitoperation at a normal condition;

[0029] FIGS. 6A-6F are waveform charts for explaining the circuitoperation at an emitter depletion condition;

[0030]FIG. 7 is a schematic circuit diagram showing a discharge lampdriving device in accordance with a second embodiment of the presentinvention;

[0031]FIG. 8 is a circuit diagram of a principal portion of the abovedevice;

[0032]FIG. 9 is a schematic circuit diagram showing a discharge lampdriving device in accordance with a third embodiment of the presentinvention;

[0033]FIG. 10 is a circuit diagram of a principal portion of the abovedevice;

[0034]FIG. 11 is a schematic circuit diagram showing a discharge lampdriving device in accordance with a fourth embodiment of the presentinvention;

[0035]FIG. 12 is a circuit diagram of a principal portion of the abovedevice;

[0036]FIG. 13 is a schematic circuit diagram showing a discharge lampdriving device in accordance with a fifth embodiment of the presentinvention;

[0037]FIG. 14 is a circuit diagram of a principal portion of the abovedevice;

[0038]FIG. 15 is a partly omitted schematic circuit diagram showing adischarge lamp driving device in accordance with a sixth embodiment ofthe present invention;

[0039]FIG. 16 is a partly omitted schematic circuit diagram showing adischarge lamp driving device in accordance with a seventh embodiment ofthe present invention;

[0040]FIG. 17 is a partly omitted schematic circuit diagram showing adischarge lamp driving device in accordance with an eighth embodiment ofthe present invention;

[0041]FIG. 18 is a circuit diagram of a principal portion of the abovedevice;

[0042]FIG. 19 is a schematic circuit diagram showing a discharge lampdriving device in accordance with a ninth embodiment of the presentinvention;

[0043]FIG. 20 is a schematic circuit diagram showing a modification ofthe above device;

[0044]FIG. 21 is a schematic circuit diagram showing anothermodification of the above device;

[0045]FIG. 22 is a schematic circuit diagram showing a furthermodification of the above device; and

[0046]FIG. 23 is a schematic circuit diagram showing a discharge lampdriving device in accordance with a tenth embodiment of the presentinvention.

BEST MODES FOR CARRYING OUT THE INVENTION

[0047] (First Embodiment)

[0048]FIG. 3 shows a schematic circuit diagram of the discharge lampdriving device in accordance with the present embodiment. A seriesconnected pair of switching elements Q1 and Q2 and a smoothing capacitorC0 are connected in parallel across pulsating output terminals of arectifier DB that is composed of a diode bridge to rectify an AC sourcevoltage AC. A leakage transformer LT1 has a primary winding N1 which isconnected between the high-side output terminal of the rectifier DB anda connection point of the switching elements Q1 and Q2, and has asecondary winding N2 connected to filaments (a) and (d) of dischargelamps La1 and La2 of the same rating. For supplying a pre-heatingcurrent, the leakage transformer LT1 has an auxiliary winding N3 whichis connected through a DC blocking capacitor C3 to the other filaments(b) and (c) of the discharge lamps La1 and La2. A capacitor C2 isconnected to the ends of the filaments (a) and (d) away from the voltagesource such that a resonant load circuit is constituted by the leakagetransformer LT1, capacitor C2 and the discharge lamps La1 and La2.

[0049] In the present embodiment, the switching elements Q1 and Q2 iscooperative with the resonant load circuit to realize an invertercircuit INV of a half-bridge type which receives, as an input voltage,the DC voltage smoothed by the smoothing capacitor C0. The half-bridgetype inverter circuit INV is known and is driven by a driver circuit(not shown but including a self-excited type using a drivingtransformer) to turn on and off the switching elements Q1 and Q2alternately at a high frequency, applying a square wave high frequencyvoltage to the resonant load circuit so as to make the use of theresonance by a leakage inductance of the leakage transformer LT1 and theresonant-inducing capacitor C2 of the resonant load circuit to supply ahigh frequency voltage of substantially the sinusoidal waveform foroperating the discharge lamps La1 and La2.

[0050] Next, the characterizing features of the present embodiment willbe explained. Impedance elements Z1, Z1 are inserted respectivelybetween the filament (a) of the discharge lamp La1 and a node (ground)having no high frequency amplitude and between the filament (d) of thedischarge lamp La2 and the node, while an impedance element Z2 isinserted between the filament (b) and the capacitor C3 that is connectedbetween the high-side terminal of the rectifier DB and the auxiliarywinding N3 of the leakage transformer LT1. Further, a series circuit ofimpedance elements Z3 and Z4 is connected between the ground and aconnection point of the auxiliary winding N3 and the filament (c).

[0051]FIG. 4 is a circuit diagram showing the resonant load circuitextracted as a principal portion. The lamp voltages VLa1 and VLa2supplied to the two discharge lamps La1 and La2 are each applied to eachclosed loop composed of the impedance elements Z1, Z3, and Z4. Also, thepulsating output Vdc from the rectifier DB divided by the impedanceelement Z2 is applied as a DC voltage to the series circuit of theimpedance elements Z3 and Z4. A detected voltage Vk derived from theconnection point of the impedance elements Z3 and Z4 is a voltagecorresponding to a combination of an AC component which is a differencebetween the lamp voltages VLa1 and VLa2 of the two discharge lamps La1and La2 respectively divided by the impedance elements Z1, Z3 and Z4,and a DC component which is the pulsating output Vdc from the rectifierDB divided by the impedance elements Z2, Z3, and Z4.

[0052] When both of the two discharge lamps La1 and La2 are normal, thelamp voltages VLa1 and VLa2 of the lamps La1 and La2 are sinusoidal ofthe same amplitude but in out of phase relation to each other by aboutone-half cycle, as shown in FIGS. 5A and 5B, such that the lamp voltagesare cancelled at the connection point of the impedance elements Z3 andZ4, causing the detected voltage Vk to have substantially zero ACcomponent Vk(AC), as shown in FIG. 5C. In this condition, since theconnection point of the Impedance elements Z3 and Z4 sees the DCcomponent Vk(DC) depending upon the dividing ratio of the impendanceelements Z2 to Z4, as seen in FIG. 5, the detected voltage Vk iseventually equal to the DC component Vk(DC).

[0053] When, on the other hand, the filament of the discharge lamp La1becomes depleted (emitter depletion condition), for example, thefilament radiates only a reduced amount of thermion, whereby the lampvoltage VLa1 of the discharge lamp La1 be comes asymmetric with respectto the zero voltage with a larger amplitude than in the normalcondition. With this result, no cancellation of the voltages is made atthe connection point of the impedance elements Z3 and Z4, whereby anoscillation voltage appears as the AC component Vk(AC) of the detectedvoltage Vk, as shown in FIG. 6C. It is noted that the DC componentVk(DC) is kept unvaried, as shown in FIG. 6D. That is, the detectedvoltage Vk will be the voltage corresponding to the high frequency ACcomponent Vk(AC) superimposed on the DC component Vk(DC), as shown inFIG. 6E. Therefore, the detected voltage Vk, which is the high frequencyAC component Vk(AC) superimposed on the DC component Vk(DC), can beprocessed such as by a peak detection in order to obtain a purely DCdetected voltage Vk′, as shown in FIG. 6F, depending on the lamp voltageVLa1 of the discharge lamp La1 suffering from the depletion of theemitter. Thus obtained detected voltage Vk′ is compared with apredetermined threshold Vth such that the discharge lamp can be judgedto reach the lamp life end when the detected voltage Vk′ exceeds thethreshold Vth. This judgment is made at an abnormality detection circuit(not shown) which transmits an abnormality detection signal to a controlcircuit (not shown) when the abnormality (the lamp life end due to theemitter depletion condition) is detected. In response to the abnormalitysignal, the control circuit responds to control the switching elementsQ1 and Q2 in such a manner as to intermittently oscillate the invertercircuit for making the circuit protection.

[0054] The present embodiment is contemplated to insert the impedanceelements Z1, Z1 respectively between the one filament of the dischargelamp La1 and the node having no high frequency amplitude (the ground)and between the one filament of the discharge lamp La2 and the node, andto detect the AC component difference between the lamp voltages VLa1 andVLa2 of the discharge lamps La1 and La2 in the respective closed loopseach including the impedance element Z1 and each of the discharge lampsLa1 and La2 in order to judge whether or not there is the abnormalitydue to the depletion of the emitter. Therefore, it is made possible todetect the occurrence of the abnormality reliably irrespective of thefact that the discharge lamps La1 and La2 give the lamp voltages VLa1and VLa2 of varying amplitudes depending on the temperature, i.e.,irrespective of the low and high temperature environments. Also, sincethere is no need to include a DC blocking capacitor on the side of thesecondary winding N2 of the leakage transformer LT1, no DC component isapplied to the discharge lamps La1 and La2 so as to prevent thecataphoresis phenomenon. Further, since the present embodiment isconfigured such that the pulsating output Vdc of the rectifier DB has aneffect on the detected voltage Vk′, it is possible to reliably detectthe occurrence of the abnormality even with the use of the invertercircuit of which output varies with the varying AC source voltage, thatis, increases with the raised AC source voltage and decreases with thelowered AC source voltage.

[0055] (Second Embodiment)

[0056]FIG. 7 shows a schematic circuit diagram of the discharge lampdriving device in accordance with the present embodiment, while FIG. 8shows a circuit diagram about a principal portion of the above. A basicconfiguration of the present embodiment is identical to the firstembodiment and therefore no duplicate explanation is made herein. Thelike parts are designated by the like reference numerals. Here, only thecharacterizing features of the present embodiment will be now explained.

[0057] The present embodiment is contemplated to insert seriescombinations of impedance elements Z1 and Z5 respectively between thefilament (a) of the discharge lamp La1 and the ground, and between thefilament (d) of the discharge lamp La2, and also insert a impedanceelement Z3 alone between the filament (c) of the discharge lamp La2 andthe ground. The like abnormality detection circuit (not shown) isincluded to judge the presence of the abnormality with regard to the onedischarge lamp La1 based upon the detected voltage Vk1 derived from theconnection point of the impedance elements Z1 and Z5, and to judge thepresence of the abnormality with regard to the other discharge lamp La2based upon the detected voltage Vk2 derived from the connection point ofthe impedance elements Z1 and Z6. When the abnormality is judged froanyone of the discharge lamps La1 and La2, the like control circuit (nowshown) operates to give the protective action such as by making theintermittent oscillation.

[0058] In the present embodiment, the detected voltage Vk1 reflectingthe lamp voltage VLa1 of the discharge lamp La1 is used to detect theabnormality (depletion of the emitter), and the detected voltage Vk2reflecting the lamp voltage VLa2 of the discharge lamp La2 is used todetect the abnormality (depletion of the emitter). Also in the presentembodiment, it is equally possible to reliably judge the abnormalityirrespective of the varying amplitudes of the lamp voltage VLa1 and VLa2from low to high temperature environments as is made in the firstembodiment. Also, since the detected voltages Vk1 and Vk2 are madereflective of the DC component of the pulsating output Vdc from therectifier DB as is made in the first embodiment, it is possible toreliably detect the occurrence of the abnormality even with the use ofthe inverter circuit of which output varies with the varying AC sourcevoltage, that is, increases with the raised AC source voltage anddecreases with the lowered AC source voltage.

[0059] (Third Embodiment)

[0060]FIG. 9 shows a schematic circuit diagram of the discharge lampdriving device in accordance with the present embodiment, while FIG. 10shows a circuit diagram about a principal portion of the above. A basicconfiguration of the present embodiment is identical to the firstembodiment and therefore no duplicate explanation is made herein. Thelike parts are designated by the like reference numerals. Here, only thecharacterizing features of the present embodiment will be now explained.

[0061] The present embodiment is characterized to insert a seriescombination of impedance elements Z1 and Z5 between the filament (a) ofthe discharge lamp La1 and the ground in order to obtain a detectedvoltage Vk1 derived from the connection point between the impedanceelements Z1 and Z5, and to obtain a detected voltage Vk2 derived fromthe connection point between impedance elements Z3 and Z4 such that thelike abnormality detection circuit (not shown) can judge the occurrenceof the abnormality for the discharge lamps La1 and La2 based upon thedetected voltages Vk1 and Vk2. When the abnormality is judged to occurin the discharge lamps La1 and La2, the like control circuit (not shown)operates to give the protective action such as by making theintermittent oscillation. In the first embodiment, either when thereoccurs the depletion of the emitter in the filament (a) of the dischargelamp La1 connected to the secondary winding N2 and also in the filament(c) of the discharge lamp La2 connected to the auxiliary winding, orwhen there occurs the depletion of the emitter in the filament (b) ofthe discharge lamp La1 connected to the auxiliary winding N3 and also inthe filament (d) of the discharge lamp La2 connected to the secondarywinding N2, the detected voltage Vk has only a small AC component Vk(DC)which makes it difficult to judge the presence of the abnormality.

[0062] However, in the present embodiment, the detected voltage Vk2derived from the connection point of the impedance elements Z3 and Z4 isrelied upon to judge whether anyone of the discharge lamps La1 and La2reaches the lamp life end due to the depletion of the emitter, while thedetected voltage Vk1, which is derived from the connection point of theimpedance elements Z1 and Z5 as corresponding to the lamp voltage VLa1of the discharge lamp La1, is relied upon to judge whether both of thedischarge lamps La1 and La2 reach the lamp life end due to the depletionof the emitter. That is, the lamp life end can be judged even in acondition which satisfies both of the events, one in which the depletionof the emitter occurs in the filament (a) of the discharge lamp La1connected to the secondary winding N2 or in the filament (c) of thedischarge lamp La2 connected to the auxiliary winding, and the other inwhich the depletion of the emitter occurs in the filament (b) of thedischarge lamp La1 connected to the auxiliary winding N3, or in thefilament (d) of the discharge lamp La2 connected to the secondarywinding N2.

[0063] (Fourth Embodiment)

[0064]FIG. 11 shows a schematic circuit diagram of the discharge lampdriving device in accordance with the present embodiment, while FIG. 12shows a circuit diagram about a principal portion of the above. A basicconfiguration of the present embodiment is identical to the firstembodiment and therefore no duplicate explanation is made herein. Thelike parts are designated by the like reference numerals. Here, only thecharacterizing features of the present embodiment will be now explained.

[0065] The present embodiment, which combines the features of the firstembodiment and the second embodiment, is characterized to insert aseries circuit of impedance elements Z1 and Z5 between the filament (a)of the discharge lamp La1 and the ground, and another series circuit ofimpedance elements Z1 and Z6 between the filament (d) of the dischargelamp La2 and the ground, and to utilize the like abnormality detectioncircuit (not shown) which judges the abnormality in either or both ofthe discharge lamps La1 and La2 based upon a detected voltage Vk1derived from the point to the impedance elements Z2 and Z5 ascorresponding the lamp voltage VLa1 of the discharge lamp La1, upon adetected voltage Vk2 derived from the connection point of the impedanceelements Z3 and Z4, and upon a detected voltage Vk3 derived from theconnection point of the impedance elements Z1 and Z6 as corresponding tothe lamp voltage VLa2 of the discharge lamp La2.

[0066] With the present embodiment, it is possible to judge theabnormality in all events including the depletion of the emitter inanyone of the discharge lamps but also in both of the discharge lampsLa1 and La2.

[0067] (Fifth Embodiment)

[0068]FIG. 13 shows a schematic circuit diagram of the discharge lampdriving device in accordance with the present embodiment, while FIG. 14shows a circuit diagram about a principal portion of the above. A basicconfiguration of the present embodiment is identical to the firstembodiment and therefore no duplicate explanation is made herein. Thelike parts are designated by the like reference numerals. Here, only thecharacterizing features of the present embodiment will be now explained.

[0069] The present embodiment utilizes capacitors C101 and C102 as theindividual impedance elements Z1 and Z1, and a resistor 109 connectedbetween the capacitors C101, C102 and the ground. The resistor 109limits a high frequency signal flowing through capacitors C101 and C012to the ground in the normal operating condition of the discharge lampsLa1 and La2, reducing circuit noises. An inductor may be utilizedinstead of resistor 109.

[0070] Also, a peak detection circuit P is provided to convert thedetected voltage Vk at the connection point of the impedance elements Z3and Z4 respectively in the form of resistors R101 and R102 into adetected DC voltage Vk′. The peak detection circuit P includes a seriescircuit of a DC blocking capacitor C401 and a diode D402 connected tothe point between the resistors R101 and R102, a diode D401 insertedbetween the ground and the connection point of capacitor C401 and diodeD401, and a smoothing capacitor C402 connected between the cathode ofdiode D402 and the ground. Thus, the capacitor C401 DC cuts out the DCcomponent Vk(DC) of the detected voltage Vk so as to charge C402 withenergy corresponding to the peak value of the AC component Vk(AC) of thedetected voltage Vk, thereby effectively obtaining the detected voltageVk′ having only the DC component corresponding to the difference in thelamp voltages VLa1 and VLa2 of the discharge lamps La1 and La2. As isexplained with reference to the first embodiment, the detected voltageVk′ is compared with the predetermined threshold Vth such that thedischarge lamps La1 and La2 can be judged to reach the lamp life endwhen the detected voltage exceeds the threshold Vth.

[0071] (Sixth Embodiment)

[0072]FIG. 15 shows a schematic circuit diagram of the discharge lampdriving device in accordance with the present embodiment. A basicconfiguration of the present embodiment is identical to the fifthembodiment and therefore no duplicate explanation is made herein. Thelike parts are designated by the like reference numerals. Here, only thecharacterizing features of the present embodiment will be now explained.

[0073] The present embodiment is characterized in that capacitors C501and C502 are utilized respectively as impedance elements Z1 and Z1, andalso act as the resonant inducing capacitor C2 to dispense withcapacitor C2. The circuit operation such as for detecting the depletionof the emitter is identical to the fifth embodiment and therefore itsexplanation is not made herein.

[0074] Thus, the present embodiment has an advantage of reducing thenumber of the components as the capacitors C501 and C502 are utilized asthe impedance elements Z1 and also as the resonant inducing capacitorC2.

[0075] (Seventh Embodiment)

[0076]FIG. 16 is a partially omitted schematic circuit diagram showingthe present embodiment which is basically similar to the second priorart of FIG. 2. Therefore, like configuration common to the second priorart is not shown and no duplicate explanation is made herein. Like partsare designated by like reference numerals. Here, only the characterizingfeatures of the present embodiment will be now explained.

[0077] As shown in FIG. 16, a resistor R1 is inserted between thehigh-side output terminal of the rectifier DB and a connection point ofcapacitor C6 connected to the auxiliary winding N3 of the leakagetransformer LT1 and one filament (b) of the discharge lamp La1. Aparallel circuit of a capacitor C8 and a resistor R5 is connected inseries with resistors R3 and R4 between the ground and the connectionpoint of the auxiliary winding N3 and the filament (c) of the dischargelamp La2. Further, the switching element Q2 has its gate connectedthrough a triggering element TD such as Diac to the connection point ofresistor R4 and capacitor C8, while a series circuit of a diode D11 anda resistor R10 is inserted between the drain of the switching element Q2and the connection point of resistor R4 and capacitor C8. A seriescombination of the triggering element TD, diode D11 and resistor RIOconstitutes a starting circuit for turning on the switching element Q2when the AC source voltage AC is applied so as to start the inverter.The like peak detection circuit P as explained with reference to thefifth embodiment is connected to the point between resistors R3 and R4to derive a detected voltage Vk from the connection point.

[0078] When the AC source voltage is applied, the rectifier DB chargecapacitor C8 through resistor R1, filament (b) of discharge lamp La1,filament (c) of discharge lamp La2, and resistors R3 and R4. Whenvoltage across capacitor C8 increases to the break voltage of thetriggering element TD, the triggering element responds to break-down forsupplying the charge of capacitor C8 to the gate of switching elementQ2, thereby turning on switching element Q2 and therefore starting theinverter circuit. When the switching element Q2 is turned on, capacitorC8 is discharged through diode D11, resistor R10 and switching elementQ2 so that the inverter circuit continues to oscillate. If the filament(b) of the discharge lamp La1 or the filament (c) of the discharge lampLa2 is broken, or if anyone of the discharge lamps La1 and La2 isdisconnected (in no-load condition) at the time of emerging the device,no charging path is established for capacitor C8. Consequently, in viewof that capacitor C8 is shunt by resistor R5, the triggering element TDwould not break-down and therefore the inverter circuit would not start.Thus, the inverter circuit is prevented from starting at the no-loadcondition for protection of the circuit at the no-load condition.

[0079] As explained in the above, since the starting circuit for theinverter circuit of the present embodiment includes the no-loaddetecting and circuit protective function of dealing with the brokenfilaments and the disconnection of the discharge lamps La1 and La2, inaddition to the abnormality detection and protection function of dealingwith the depletion of the emitter, the circuit components can be reducedsignificantly in number.

[0080] (Eighth Embodiment)

[0081]FIG. 17 shows a schematic circuit diagram of the discharge lampdriving device in accordance with the present embodiment, while FIG. 18shows a circuit diagram about a principal portion of the above. A basicconfiguration of the present embodiment is identical to the second priorart of FIG. 2 as well as to the seventh embodiment. Therefore noduplicate explanation is made herein but the like parts are designatedby the like reference numerals. Here, only the characterizing featuresof the present embodiment will be now explained.

[0082] The present embodiment is configure to insert impedance elementsZ1 and Z1 respectively between the filament (a) of the discharge lampLa1 and the ground and between the filament (c) of the discharge lampLa2, and insert a series combination of impedance elements Z3 and Z4between the ground and the connection point of the auxiliary winding N3and the filament (c) of the discharge lamp La2. Also, the like peakdetection circuit P as explained with reference to the fifth embodimentis connected to a connection point of the impedance elements Z3 and Z4so that the detected voltage Vk derived from the connection point of theimpedance elements Z3 and Z4 is converted into a DC detected voltageVk′.

[0083] The control circuit CNT compares the detected voltage Vk′ fromthe peak detection circuit P with a predetermined threshold Vth so as tojudge that the discharge lamp La1 or La2 reaches the lamp life end whenthe threshold Vth is exceeded, and makes the protective action ofintermittently oscillating the inverter circuit.

[0084] Thus, in the like manner as in the first embodiment, the presentembodiment includes the impedance elements Z1 and 72 which are insertedrespectively between the one filament of the one discharge lamp La1 andthe node having no high frequency amplitude (the ground), and betweenthe one filament of the other discharge lamp La2 and the ground, inorder to detect a difference in the AC component of the lamp voltagesVLa1 and VLa2 of the discharge lamps La1 and La2 within the closed loopseach including the impedance element Z1 and each of the discharge lampsLa1 and La2, for the purpose of judging the abnormality due to thedepletion of the emitter. Accordingly, it can be made to reliably judgethe abnormality irrespective of the varying amplitudes of the lampvoltage VLa1 and VLa2 from low to high temperature environments. Also,since there is no need to include a DC blocking capacitor on the side ofthe secondary winding N2 of the leakage transformer LT1, no DC componentis applied to the discharge lamps La1 and La2 so as to prevent thecataphoresis phenomenon.

[0085] (Ninth Embodiment)

[0086]FIG. 19 shows a schematic circuit diagram of the discharge lampdriving device in accordance with the present embodiment. The presentembodiment includes a rectifier DB in the form of a diode bridgeresponsible for a full-wave rectification of an AC source voltage AC toprovide a pulsating output that is smoothed by a smoothing capacitor C1to give a voltage source for an inverter circuit. The inverter circuitis of a so-called half-bridge configuration and includes a seriescombination of switching elements Q1 and Q2 respectively in the form ofbipolar transistors connected across the smoothing capacitor C1, diodesD1 and D2 each connected in anti-parallel relation across each of theswitching elements Q1 and Q2, and a series circuit of capacitors C3 andC4 connected across the smoothing capacitor C1. Connected to a pointbetween capacitors C3 and C4 is a series circuit of a primary winding N1of a leakage transformer LT1 and a primary winding of a drivingtransformer T1 which is provided for driving the switching elements Q1and Q2. The leakage transformer LT1 has a secondary winding N2 connectedto filaments (a) and (d) of the discharge lamps La1 and La2, and anauxiliary winding N3 connected to filaments (b) and (c) of the dischargelamps La1 and La2. A resonant inducing capacitor C5 is connected tofilaments (a) and (d) of the discharge lamps Lal and La2 on thenon-energized side thereof. Instead of using the combination of thebipolar transistors and the diodes D1 and D2, the switching elements Q1and Q2 may be realized by field effect transistors having parasiticdiodes.

[0087] The switching elements Q1 and Q2 are activated by the drivingtransformer T1 to turn on and off alternately with the switchingelements Q1 and Q2 being responsible for flowing currents in oppositedirections through the leakage transformer LT1 to the discharge lampsLa1 and La2 respectively from capacitors C3 and C4, thereby applying ahigh frequency voltage developed across capacitor C5 resulting from aseries resonant circuit of a leakage inductance and capacitor C5 forstarting and operating the lamps.

[0088] Also in the present embodiment, a capacitor C8 is inserted as theimpedance element between the filament (a) of the discharge lamp La1 anda node (the ground) having no high frequency amplitude, while acapacitor C9 is inserted as the impedance element between the filament(b) of the discharge lamp La2 and the node (the high-side outputterminal of rectifier DB). Further, an emitter depletion detection andprotection circuit 10 is connected between a connection of a baseresistor R2 of the switching element Q2 with the secondary winding ofthe driving transformer T1 and the auxiliary winding N3 in order todetect the depletion of emitter in anyone of the filaments (a) to (d) ofthe discharge lamps La1 and La2 for protection of the circuit.

[0089] The emitter depletion detection and protection circuit 10includes a series circuit of a DC blocking capacitor C7 and a diode D6connected between the filament (c) of the discharge lamp La2 and theground, a diode D5 having an anode connected to a cathode of diode D6connected to the capacitor C7, a zener diode ZD1 having a cathodeconnected to the cathode of diode D5, and a parallel combination of asmoothing capacitor C6 and a discharging resistor R5 connected betweenthe cathode of zener diode ZD1 and the ground. A capacitor C10 isconnected in parallel with a biasing resistor R4 between the anode ofzener diode ZD1 and the ground, while a switching element Q3 of PNP-typebipolar transistor is connected in series with a diode D7 between thebase resistor R2 of the switching element Q2 and the resistor R4.Further, a biasing resistor R3 is connected in an emitter-base path ofthe switching element Q3, while a switching element Q4 of NPN-typebipolar transistor is connected between resistor R3 and the switchingelement Q4.

[0090] As the capacitor C8 is inserted between the filament (a) of thedischarge lamp La1 and the ground and the capacitor C9 is insertedbetween the filament (d) of the discharge lamp La2 and the high-sideoutput terminal of the rectifier DB, the high frequency currentsrespectively flowing through the discharge lamps La1 and La2 becomesasymmetrical with each other if anyone of the filaments (a) to (d) ofthe discharge lamps La1 and La2 sees the depletion of the emitter. Theresulting asymmetrical high frequency currents are responsible forcharging the capacitor C7 and the capacitor C6 through diode D5. Whenthe voltage across capacitor C6 exceeds the zener voltage of zener diodeZD1, capacitor C6 is discharged to turn on the switching element Q4,which in turn causes the switching element Q3 to turn on, therebyconnecting the secondary winding of the driving transformer T1 fordriving the switching element Q2 to the ground through diode D7. Withthis result, the switching element Q2 becomes not capable of turning onto stop the inverter circuit. Thus, the emitter depletion detection andprotection circuit 10 can detect the depletion of the emitter of thedischarge lamps La1 and La2, and stops the inverter circuit forprotection of the circuit upon detection of the depletion of theemitter.

[0091] In the present embodiment, the impedance elements C8 and C9 areinserted respectively between the filaments of the discharge lamps La1,La2 and the nodes having no high frequency amplitude (the ground or thehigh-side output terminal of rectifier DB) in order to detect theasymmetric high frequency currents at the connection between thedischarge lamps La1 and La2 for judging whether there occurs thedepletion of emitter. Therefore, it is possible to reliably judge theoccurrence of the depletion of the emitter irrespective of whether it isoperating in the low or high temperature environment. Further, sincethere is no need to connect a DC blocking capacitor to the secondarywinding N2 of the leakage transformer LT1, the discharge lamps La1 andLa2 can be free from the DC component so as to be prevented from causingthe cataphoresis phenomenon.

[0092] It may be equally possible to insert capacitors C8 and C9respectively between the filament (a) of the discharge lamp La1 and thehigh-side output terminal of the rectifier DB, and between the filament(d) of the discharge lamp La2 and the high-side output terminal, asshown in FIG. 20; to insert capacitors C8 and C9 respectively betweenthe filament (a) of the discharge lamp La1 and the ground, and betweenthe filament (d) of the discharge lamp La2 and the ground, as shown inFIG. 21; to insert resistors Ra and Rd instead of capacitors C8 and C8between the respective filaments (a) and (d) of the discharge lamps La1and La2 and the respective one of the high-side output terminal ofrectifier DB and the ground, as shown in FIG. 22; or even to use aseries combination of resistor and capacitor as the impedance element.In any case, the high frequency currents flowing through the dischargelamps La1 and La2 becomes asymmetrical with each other when there occursthe depletion of the emitter in anyone of the filaments (a) to (d) ofthe discharge lamps La1 and La2 so that the emitter depletion detectionand protection circuit 10 can responds to detect the asymmetrical highfrequency currents for judging whether or not there occurs the depletionof the emitter.

[0093] (Tenth Embodiment)

[0094]FIG. 23 shows a schematic circuit diagram of the discharge lampdriving device in accordance with the present embodiment which isbasically similar to the second prior art of FIG. 2. Therefore, likeconfiguration common to the second prior art is not shown and noduplicate explanation is made herein. Like parts are designated by likereference numerals. Here, only the characterizing features of thepresent embodiment will be now explained.

[0095] In the present embodiment, capacitor C8 is inserted as theimpedance element between the filament (a) of the discharge lamp La1 andthe node (the high-side output terminal of rectifier DB) having no highfrequency amplitude, while capacitor C9 is inserted as the impedanceelement between the filament (d) of the discharge lamp La2 and the node(ground). Also, connected between the gate of the switching element Q2and the auxiliary winding N3 is the like emitter depletion detection andprotection circuit 10 which detects the depletion of the emitter inanyone of the filaments (a) to (d) of the discharge lamps La1 and La2for protection of the circuit. The emitter depletion detection andprotection circuit is identical in configuration and operation to thatof the ninth embodiment, and therefore no duplication explanation ismade.

[0096] Similar to the ninth embodiment, the present embodiment isconfigured to insert capacitor C8 between the filament (a) of thedischarge lamp La1 and the high-side output terminal of rectifier DB, toinsert capacitor C9 between the filament (d) of the discharge lamp La2and the ground, and to provide the emitter depletion detection andprotection circuit 10 which detects the asymmetric high frequencycurrents at the connection between the discharge lamps La1 and La2 forjudging whether there occurs the depletion of the emitter. Therefore, itis possible to reliably judge the occurrence of the depletion of theemitter irrespective of whether it is operating in the low or hightemperature environment. Further, since there is no need to connect a DCblocking capacitor to the secondary winding N2 of the leakagetransformer LT1, the discharge lamps La1 and La2 can be free from the DCcomponent so as to be prevented from causing the cataphoresisphenomenon.

[0097] The inverter circuit may be of different circuit configurationsincluding, for example, one in which the resonant load circuit isconnected between the connection point of the switching elements Q1 andQ2 and the low-side output terminal of the rectifier DB, and one inwhich a valley-filling power source composed of a voltage doubler isutilized instead of the valley-filling power source composed of thestep-down chopper circuit. The concept of the present invention can beapplied

[0098] It is noted that the concept of the present invention can beapplied to various circuit configurations of the inverter circuit. Forexample, the inverter circuit may be of different configurationsincluding one in which the resonant load circuit is connected betweenthe connection point of the switching elements Q1 and Q2 and thelow-side output terminal of the rectifier DB, and one in which avalley-filling power source composed of a voltage doubler is utilizedinstead of the valley-filling power source composed of the step-downchopper circuit.

1. A discharge lamp driving device comprising: a rectifier whichrectifies an AC source voltage; a smoothing capacitor which smoothes outa pulsating output of the rectifier; an inverter circuit having at leastone switching element for conversion of the smoothed DC output madethrough the smoothing capacitor into a high frequency output; a loadcircuit including a resonance circuit and a discharge lamp and beingsupplied with the high frequency output from the inverter circuit; anoutput transformer having a primary connected to an output end of theinverter circuit and having a secondary connected to one filament end ofthe discharge lamp; an impedance element inserted respectively betweenthe other filament end of the discharge lamp and a node having no highfrequency amplitude; and an abnormality detection and protection meanswhich detects an amplitude of the high frequency output flowing throughthe discharge lamp and the impedance element in order to make a circuitprotection when the detected amplitude exceeds a predeterminedthreshold.
 2. The discharge lamp driving device as set forth in claim 1,wherein said impedance element is inserted between said other filamentend of the discharge lamp and a positive side input terminal of theinverter circuit.
 3. The discharge lamp driving device as set forth inclaim 1, wherein said impedance element is inserted between said otherfilament end of the discharge lamp and a grounded input terminal oroutput terminal of the inverter circuit.
 4. The discharge lamp drivingdevice as set forth in anyone of claims 1 to 3, wherein a plurality ofsaid discharge lamps are connected in series across the secondary ofsaid output transformer.
 5. The discharge lamp driving device as setforth in claim 4, wherein the impedance elements inserted between thefilaments of the respective discharge lamps and the node having no highfrequency amplitude have substantially the same impedance value.
 6. Thedischarge lamp driving device as set forth in claim 1, wherein aplurality of said discharge lamps are connected in series across thesecondary of said output transformer, the impedance element beinginserted between the other filament of at least one of said dischargelamps and the positive side input terminal of the inverter circuit, andanother impedance element being inserted between the other filament endof at least another said discharge lamp and the grounded input terminalor output terminal of the inverter circuit.
 7. The discharge lampdriving device as set forth in claim 1, wherein a plurality of saiddischarge lamps are connected in series across the secondary of saidoutput transformer, said abnormality detection and protection meansmaking the circuit protection when the amplitude of the high frequencyoutput flowing at least one of said discharge lamps and the impedanceelement exceeds a predetermined threshold.
 8. The discharge lamp drivingdevice as set forth in claim 1, wherein a plurality of said dischargelamps are connected in series across the secondary of said outputtransformer, said abnormality detection and protection means detectingthe amplitude of the voltage at the connection between the filaments ofthe individual discharge lamps, detecting the amplitude of the highfrequency output flowing through at least one of said discharge lampsand the impedance element, and making the circuit protection when atleast one of said amplitudes exceeds a predetermined threshold.
 9. Thedischarge lamp driving device as set forth in claim 1, wherein aplurality of said discharge lamps are connected in series across thesecondary of said output transformer, said abnormality detection andprotection means detecting the amplitude of the voltage at theconnection between the filaments of the individual discharge lamps, andmaking the circuit protection when at the amplitude at said connectionor an amplitude of a high frequency output flowing through at least oneof the high-voltage and low-voltage side discharge lamps and through theimpedance element exceeds a predetermined threshold.
 10. The dischargelamp driving device as set forth in claim 1, wherein said impedanceelement is a resistor.
 11. The discharge lamp driving device as setforth in claim 1, wherein said impedance element is a capacitor.
 12. Thedischarge lamp driving device as set forth in claim 1, wherein saidimpedance element is a series combination of a resistor and a capacitor.13. The discharge lamp driving device as set forth in claim 1, whereinsaid inverter circuit is of a self-excited type, and a starting circuitfor starting the inverter circuit shares at least a portion thereof withsaid abnormality detection and protection means.
 14. The discharge lampdriving device as set forth in claim 1, wherein said impedance elementis shared with a resonant circuit included in the load circuit.